Renewable energy sources, such as solar and wind farms, are becoming more economically viable as sources of energy. Renewable energy plants typically include a plurality of power converters configured to convert power generated by the renewable energy source(s) (e.g. photovoltaic arrays, wind turbines, etc.) to AC power suitable for a utility grid. For example, a typical solar farm will include a plurality of inverters configured to convert DC power generated by one or more photovoltaic arrays (PV arrays) to suitable AC power at a grid frequency for application to the utility grid. The plurality of inverters can be coupled to the grid through a transformer at a point of interconnection for the renewable energy plant.
Voltage regulation at the point of interconnection for a renewable energy plant, such as a solar farm, can be accomplished by regulating the reactive power output of the renewable energy plant. For instance, a control system can provide reactive power commands (Q commands) to each of the plurality of inverters in the renewable energy plant to control the reactive power output of the individual inverters to achieve a desired reactive power output at the point of interconnection for the renewable energy plant.
In grid transmission applications (e.g. high voltage transmission applications), reactive power output at the point of interconnection with the grid transmission system can have an influence on voltage at the point of interconnection. Because most transmission systems are inductive, increasing reactive power can typically cause the voltage at the point of interconnection to increase. Conversely, decreasing a reactive power output can typically cause the voltage at the point of interconnection to decrease.
Regulating voltage at the point of interconnection in grid distribution applications (e.g. medium voltage or less applications) can be more difficult. Because of the decreased X/R ratios (reactance/resistance ratios) associated with distribution systems, increased reactive power is required to regulate voltage at the point of interconnection with a distribution application when compared to transmission applications. As a result, a control system for the renewable energy plant can attempt to output too much reactive power from each of the plurality of inverters to regulate the voltage, causing each of the plurality of inverters to quickly reach maximum output reactive power capability. In addition, the control system can fight other utility voltage regulation devices, such as tap changing transformers, rendering the control system and the other voltage regulation devices ineffective.
Thus, a need exists for an improved system and method for regulating voltage at a point of interconnection of a renewable energy plant with a grid. A system and method that regulates voltage by controlling reactive power output without adversely interacting with other voltage regulation devices, such as tap changing transformers, shunt capacitors, etc., would be particularly useful.